This is the author's final draft of the version published as FEBS Letters, 2008, 582, (2008), pp.1651-1656. The online version can also be accessed via http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T36-4SBHBHT-C&_user=123215&_coverDate=05%2F28%2F2008&_rdoc=4&_fmt=high&_orig=browse&_srch=doc-info(%23toc%234938%232008%23994179987%23690098%23FLA%23display%23Volume)&_cdi=4938&_sort=d&_docanchor=&_ct=26&_acct=C000010181&_version=1&_urlVersion=0&_userid=123215&md5=3371b907fc79bdf84ebf59e7604416f8